The technique of vacuum brazing is used to fabricate electron tubes and vacuum tubes. If a brazing filler material having high vapor pressure is exposed to the vacuum brazing atmosphere, the vapor fouls the area to be brazed as well as the inner wall of the vacuum furnace. The brazed part is also exposed to high temperatures during the service of the tubes, and the filler material is evaporated to give a vapor that not only reduces the degree of vacuum in the tubes but also fouls their interior. For these reasons, an alloy having low vapor pressure is used as the brazing filler material. Typical filler materials having low vapor pressure include an Ag-Cu alloy filler material (silver brazing filler material BAg-8 of JIS Z 3261) consisting of 27-29% Cu with the balance being Ag and incidental impurities and an Ag-Cu-Sn alloy brazing filler material consisting of 59-61% Ag, 9.5-10.5% Sn, with the balance being Cu and incidental impurities (all percents used herein are by weight). One example of each type is listed in Table 26.1 on page 297 of "Brazing Manual" published by Kogaku Tosho Shuppan, October 1980 which is a translation of "Brazing Manual", 3rd ed., edited by American Welding Society, 1976. These conventional Ag-Cu base alloy brazing filler materials have good brazability, but their silver content is so high that with the rapid increase and considerable fluctuation in the price of silver in recent years, economic considerations put rigid limits on expanding the applications of these filler materials. Attempts have been made to reduce the silver content, but this results in reduced wettability and increased brazing temperatures, which in turn impair the brazability, the cold workability in particular, of the filler materials, and the filler materials cannot be worked into a foil or very fine wire that is required in electronics to fabricate ICs and LSIs.